Study On Structural Fatigue Strength Under High-low Frequency Combined Stress

In recent years,the size of ships has become larger and larger for the sake of reduction of operating costs,the natural frequency of its structure has reached a low level(4rad/s).In the response spectrum,there are obvious double peaks,which greatly increase the stress range of the structure and cause serious fatigue damage.The paper mainly focus on the fatigue strength under high-low frequency combined stress.The high-low frequency load forecasting method of hull structure is studied.The fatigue strength evaluation method under high-low frequency combined stress is established.The main research contents of the paper are as follows:1)The torsion characteristic parameters of container ship profile studied.The container ship is regard as the groove thin-wall beam.Considering the distribution law of bending shear stress of the groove thin-wall beam section,the bending center and torsion center of the section of the groove thin-wall beam section are deduced.The transport matrix method and finite element method are used to calculate the mode of hull structure respectively,and the differences of the two results are analyzed.2)The calculation method of high-low frequency load is studied.Based on three-dimensional hydro-elastic theory,load characteristics of the hull is studied.In order to ensure the accuracy of load numerical simulation,the grid convergence of hydrodynamic coefficient and high-low frequency load is analyzed.The influence of damping coefficient,speed,wave direction and other factors on high-low frequency load is discussed.3)The fatigue strength assessment frequency and time domain methods of hull structure under high-low frequency combination stress are studied.The paper presents a combined stress fatigue assessment time domain analysis method based on the equivalent double frequency wave model,and analyzes the frequency characteristics and amplitude characteristics of high-low frequency stress components.By converting the load response in regular wave RAO into specific sea condition to the irregular wave load response.Regarding the fatigue damage result of the rain flow count method as the standard,result error analysis of the equivalent double frequency wave model and spectrum analysis method are conducted.Based on the equivalent double frequency wave model and the finite element method,a method for evaluating the hull structural fatigue considering the torsional load is presented.The effect of torsional load on fatigue strength of container ship is analyzed.4)The fatigue characteristics of a 20000 TEU container ship is studied.The stress concentration degree of container ship is analyzed,and the efficiency of wave spectrum is clear.In the paper,the fatigue damage under high-low frequency stress components is calculated by the frequency domain spectrum analysis method,and the influence of the selection method of frequency point on the structural fatigue damage is analyzed.At the same time,the fatigue damage indifference in stress component,speed,load component,wave direction and other calculation parameters are analyzed.